check语句为属性添加域即范围
    check(条件)
 
create table Student
(
	Sno char(5) ,
	Sname char(20),
	Sage smallint check(Sage>=15 and Sage<=45),
    Sage属性必须大于15，小于45
	Ssex char(2),
	primary key(Sno));
 
create table Course
(
	Cno char(2) primary key,
	Cname char(20),
    semester char(20)
	Cpno char(2),
	Ccredit smallint,
	foreign key (Cpno) references Course(Cno)
    check(semester in ('Fall','Spring','Summer','Winter')));
 
check也可放在最后，这里表示semester属性只能是以上括号内的四个字符串